Measuring absolute static pressure at one or more positions along a microfluidic device

ABSTRACT

A method of measuring absolute static pressure at one or more positions along the wall of a microfluidic device transporting a working fluid that is immiscible in a first selected gas environment, includes providing a first fluid conducting channel having an atmosphere provided by the selected gas environment under a sealed environment and in communication with the microfluidic device at a first point of communication; providing a visual scale adjacent to the first fluid conducting channel; and transporting the working fluid under pressure conducted by the microfluidic device into the first fluid conducting channel such that the volume transported into such first fluid conducting channel varies depending upon the absolute static pressure of the working fluid at the first point of communication, whereby the absolute static pressure at the first point of communication is visually determined depending on the position of the interface of the working fluid and the first selected gas environment in the first fluid conducting channel when compared with the first visual scale.

FIELD OF THE INVENTION

The present invention relates to measuring the absolute static pressure of fluid flowing in a microfluidic device.

BACKGROUND OF THE INVENTION

Conducting of fluid in microfluidic devices is frequently done in inkjet printers. It is also done in other types of microfluidic devices in fields not related to printing, for example, drug delivery and microscale biological and chemical analysis. In the inkjet printing field, ink is transported from a reservoir to a printhead, where it is sprayed onto print media. If a nozzle becomes clogged, the absolute static pressure within the relevant ejection chamber may exceed normal values. Thus, absolute static pressure detection can be used as a nozzle-by-nozzle diagnostic tool in the case of an inkjet printer. Similarly, absolute static pressure sensing in drug delivery systems is critical due to the potential hazards of inaccurate dosing. In microscale biological and chemical analysis systems, where precision is critical, an integrated absolute static pressure sensor reduces the error associated with sensing the absolute static pressure across an intervening volume between the microfluidic device and a non-integrated sensor; in addition, an integrated pressure sensor reduces the volume of fluid that must be sacrificed to measure the pressure. This increases cost and waste, and in the case of biological analysis systems, for example, it increases the required sample size, making sample collection and preparation more difficult and costly.

While prior art exists for pressure drop measurement across microfluidic and macrofluidic systems, even microscale measurement systems are integrated on the macroscale, allowing measurement of absolute static pressure as the working fluid enters or exits a microfluidic device, but not while the working fluid is within the interior of the microfluidic device. For example, U.S. Pat. No. 4,426,768 teaches the use of the measurement of a strain-dependent property to deduce the local absolute pressure of a non-conductive fluid. Similarly, U.S. Pat. No. 4,463,336 teaches the measurement of absolute gas pressure using the varying resistance of a narrow piezoresistive bridge. Electrically conductive liquids cannot be measured with this sensor. U.S. Pat. No. 4,682,503 teaches the use of measuring the varying thermal resistance of a silicon nitride bridge to the same end. This sensor does not work with an electrically conducting fluid, and struggles to provide accurate data with a fluid of high heat capacity. A number of sensors measuring pressure via the capacitance between a deforming membrane and a fixed electrode have been previously disclosed. U.S. Pat. Nos. 5,305,643; 5,316,619; 5,369,544 and 6,109,113 teach the measurement of absolute pressure against a standard (normally vacuum or the ambient atmosphere) by capacitance, while U.S. Pat. Nos. 5,332,469; 5,679,902 and 6,012,335 teach the use of capacitance to measure the relative difference of two varying pressures. In all cases, the region across which the field is applied must not be penetrated by any fluid with a different dielectric constant from that of the fluid originally between the two plates. U.S. Pat. No. 5,458,000 teaches the use of a second resonant sensor that is not affected by changes in pressure but responds to changes in temperature for use in temperature error compensation. This sensor will not operate in a viscous fluid. U.S. Pat. Nos. 5,528,939 and 5,939,635 teach the measurement of energy needed to move a resonant member near a stationary surface, where the damping losses are highly pressure dependent. This can only be used to measure a compressible working fluid, however, or the variation in damping force will not be evident. U.S. Pat. No. 5,808,210 teaches measurement of absolute pressure by measuring deformation either electrically or optically. While this sensor can be used for optical measurement of the membrane deflection, it cannot be used with an opaque working fluid.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved way to measure the absolute static pressure in a microfluidic device transporting a working fluid.

This object is achieved in a method of measuring absolute static pressure at one or more positions along the wall of a microfluidic device transporting a working fluid that is immiscible in a first selected gas environment, comprising the steps of:

(a) providing a first fluid conducting channel having an atmosphere provided by the first selected gas environment under a sealed environment and in communication with the microfluidic device at a first point of communication;

(b) providing a first visual scale adjacent to the first fluid conducting channel; and

(c) transporting the working fluid under pressure conducted by the microfluidic device into the first fluid conducting channel such that the volume transported into such first fluid conducting channel varies depending upon the absolute static pressure of the working fluid at the first point of communication, whereby the absolute static pressure at the first point of communication is visually determined depending on the position of the interface of the working fluid and the first selected gas environment in the first fluid conducting channel when compared with the first visual scale.

It is an advantage of the present invention to visually inspect the volume in a first fluid conducting channel in communication with the microfluidic device to readily determine the absolute static pressure in the first fluid conducting channel without the need for electrical elements or moving parts.

It is an advantage of the present invention to integrate the absolute static pressure sensor with nearly any existing microfluidic device without modification of the fabrication process of the microfluidic device.

It is an advantage of the present invention that the integration referred to above will not increase the fabrication cost of the pre-existing microfluidic device.

It is a further advantage of the present invention to measure absolute static pressure over a variety of pressure ranges at the same point of communication, thereby increasing the effective absolute static pressure range of the sensor.

It is a further advantage of the present invention to measure absolute static pressure at a precise location within a microfluidic device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic of an ink jet printing apparatus that has a microfluidic device in which the present invention is adapted to measure the absolute static pressure therein;

FIG. 2 is a schematic of a first embodiment of the present invention;

FIG. 3 is a schematic of a second embodiment of the present invention; and

FIG. 4 is a schematic of another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The term, “microfluidic”, “microscale” or “microfabricated” generally refers to structural elements or features of a device, such as fluid channels, chambers or conduits, having at least one fabricated dimension in the range from about 0.1 μm to about 500 μm. In devices according to the present invention, the microscale channels or chambers preferably have at least one internal cross-section dimension, e.g., depth, width, length, diameter, etc., between about 0.1 μm to about 500 μm, preferably between about 1 μm to about 200 μm.

The microfluidic devices described in the present invention are preferably fabricated with the techniques commonly associated with the semiconductor electronics industry, e.g., photolithography, dry plasma etching, wet chemical etching, etc., on the surface of a suitable substrate material, such as silicon, glass, quartz, ceramics, as well as polymeric substrates, e.g., plastics.

Various techniques using chip technology for the fabrication of microfluidic devices, and particularly micro-capillary devices, with silicon and glass substrates have been discussed by Manz, et al. (Trends in Anal. Chem. 1990, 10, 144, and Adv. In Chromatog. 1993, 33, 1). Other techniques such as laser ablation, air abrasion, injection molding, embossing, etc., are also known to be used to fabricate microfluidic devices, assuming compatibility with the selected substrate materials.

The present invention uses a method of measuring absolute static pressure at one or more positions along the wall of a microfluidic device transporting a working fluid that is immiscible in a first selected gas environment, comprising the steps of: providing a first fluid conducting channel having an atmosphere provided by the first selected gas under a sealed environment and in communication with the microfluidic device at a first point of communication; providing a visual scale adjacent to the first fluid conducting channel; and transporting the working fluid under pressure conducted by the microfluidic device into the fluid conducting channel such that the volume transported into such first fluid conducting channel varies depending upon the absolute static pressure of the working fluid at the first point of communication, whereby the absolute static pressure at the first point of communication can be visually determined depending on the position of the interface of the working fluid and the first selected gas environment in the first fluid conducting channel when compared with the first visual scale. If the volume occupied by the first selected gas environment is known at static conditions and the absolute static pressure is known under ambient conditions, then the absolute static pressure during operation can be determined by the motion of the interface between the working fluid and the first selected gas environment, using, for example, the ideal gas law,

PV=nRT,

where P represents the absolute static pressure of the first selected gas environment, which is presumed to be related in a known way to the absolute static pressure of the working fluid at the first point of communication, V represents the volume occupied by the first selected gas environment, n represents the number of moles of gas in the first selected gas environment, R represents a universal gas constant, and T represents the absolute temperature of the first selected gas environment. For steady-state working fluid/selected gas environments, the following relation applies: $P_{fluid} = {P_{gas} - {{\sigma \left( {\frac{1}{R_{1}} + \frac{1}{R_{2}}} \right)}\cos \quad \theta}}$

where P_(fluid) is the absolute static pressure of the working fluid at the point of communication; P_(gas) is the absolute static pressure of the first selected gas environment; R₁ and R₂ are the local radii of curvature of the interface between the first selected gas environment and the working fluid; θ is the contact angle of the three-phase system formed by the working fluid, the first selected gas environment, and the wall of the microfluidic device; and σ is the surface tension of the three-phase system described above. In cases where the surface tension is very small, (σ→0), the absolute static pressure of the working fluid may be considered equal to the absolute static pressure of the selected gas environment. Hence, the present invention can measure absolute static pressure at a precise location within a microfluidic device.

More complex models that relate absolute static pressure, volume, and temperature to physical constants can also be used to improve accuracy. For example, the additional terms in the van der Waals or “real gas” equation (shown below) will improve accuracy for cases where a higher density of gas molecules causes stronger intermolecular interaction. ${\left( {P + \frac{C_{1}}{V^{2}}} \right)\left( {V - C_{2}} \right)} = {nRT}$

In this equation the two additional constants C₁ and C₂ are determined by the type of gas in the first selected gas environment.

Turning now to FIG. 1, a schematic diagram is shown of a printing apparatus 10 including at least one ink reservoir 12, drive electronics 14, and a plurality of individually addressable printheads 16 in communication with both the ink reservoirs 12 and the drive electronics 14. Ink is introduced into each individually addressable printhead 16 by means of a microfluidic device 18, in this case a microfluidic channel, which in turn is in communication with an absolute static pressure sensor 20. A computer 22 receives or generates data 24 representing, for example, a digital image. The computer 22 also controls the output to the printing apparatus 10 by means of electrical signals to the drive electronics 14. The drive electronics 14 in turn control a transport mechanism 26, which conveys a receiver 28 to the individually addressable printheads 16 so that ink pixels may be transferred to the receiver 28. The ink flow may be controlled by any means including but not limited to thermal ejection, piezoelectric ejection, or electrostatic ejection. The computer 22 also controls the individually addressable printheads 16.

FIG. 2 depicts a first embodiment of the absolute static pressure sensor 20 shown in FIG. 1, that measures absolute static pressure in a working fluid 30 at the first point of communication 32 of a first fluid conducting channel 34 with the microfluidic device 18. The working fluid 30 may be, but is not limited to any of the following fluids: water, ink, ethylene glycol, methanol, ethanol, isopropanol, oil, acetone, an organic solvent, a biological fluid, a solution of a chemical analyte, or a miscible combination of these. The first point of communication 32 should be kept as small as possible to minimize disruption to the flow in the microfluidic device 18; however, if the first point of communication 32 is too small, the response time of the absolute static pressure sensor 20 will become large, since equilibration of the absolute static pressure sensor 20 will be limited by the rate at which the working fluid 30 can flow through the first point of communication 32.

In addition to the use of the present invention to measure the absolute static pressure in a microfluidic channel, the absolute static pressure sensor 20 can measure the absolute static pressure at any point in a microfluidic device 18; including, but not limited to a channel, reservoir, chamber, pump, valve, mixer, needle, or any system incorporating any combination of these. For example, the absolute static pressure of an ink chamber can be measured by placing a first fluid conducting channel 34 in communication with the ink chamber.

Generally, the placement of the sensor 20 is that the point of communication 32 should be located where the local working fluid velocity during normal operation is parallel to the plane of the point of communication 32. If the working fluid 30 can exert momentum in the direction normal to the plane of the point of communication 32, the absolute static pressure registered by the absolute static pressure sensor 20 will not be the absolute static pressure of the working fluid 30 at the first point of communication 32, but rather the absolute stagnation pressure of the working fluid 30 at the first point of communication 32, if the local fluid velocity vector is normal to the plane of the first point of communication 32, or some intermediate value between these two pressures if the local working fluid velocity vector impacts the plane of the first point of communication 32 at some angle between zero and ninety degrees.

When the individually addressable printhead 16 is primed with a working fluid 30 (in this case ink) before use by the drive electronics 14, the microfluidic device 18 will fill with fluid. Depending on the pressure used to prime the absolute static pressure sensor 20 and the ambient pressure conditions upon filling, a certain amount of gas will be trapped within a first fluid conducting channel 34, forming a first selected gas environment 36 at the end of the first fluid conducting channel 34. This first selected gas environment 36 may be air, nitrogen, argon, neon, helium, oxygen, hydrogen, water vapor, carbon monoxide, carbon dioxide or a miscible combination of these. The limitations on the selected gas environment 36 are that the gas must be immiscible with the working fluid 30 and non-reactive in the presence of the working fluid 30. Similarly, for non-inkjet microfluidic devices, there will be an initial filling of the microfluidic device 18 generally accomplished by forcing the working fluid 30 through the microfluidic device 18 using an external pressure source. In order to use the absolute static pressure sensor 20, the initial volume of this first selected gas environment 36 must be determined by optically comparing the position of a first interface 38 to a first visual scale 40 while the absolute static sensor 20 is under ambient pressure. The use of the visual scale 40 obviates the need for electrical elements or moving parts. This in turn means that the absolute static pressure sensor 20 integrates with nearly any existing microfluidic device without modification of the fabrication process of the microfluidic device. Therefore, this integration will not increase the fabrication cost of the pre-existing microfluidic device.

Subsequently, the first interface 38 position relative to the first visual scale 40 can be measured to determine the volume of the first selected gas environment 36. It is important to note that there are no restrictions on the optical properties of the working fluid 30. It can be opaque or transparent, as refraction of light near the first interface 38 will cause the interface to appear dark even if the working fluid 30 is transparent. Obviously, the first interface 38 between an opaque working fluid 30 and a transparent first selected gas environment 36 is easily optically resolvable. Using the initial volume, measured under ambient static conditions, and an equilibrium volume, measured while the microfluidic device is in operation, the operating absolute static pressure can be determined. Since a fixed amount of gas is trapped within the first selected gas environment 36, the following relation for the operational absolute static pressure, P₂, holds under isothermal equilibrium conditions as a direct result of the ideal gas law: $P_{2} = {P_{1}\frac{V_{1}}{V_{2}}}$

where P₁ is the ambient absolute static pressure (measured with, for example, an external barometer) at which V₁, the initial volume, is measured and P₂ is the operational absolute static pressure at which V₂, the operational volume, is measured. In addition to this rigorous determination of absolute static pressure under isothermal equilibrium conditions, the absolute static pressure sensor 20 can qualitatively determine the effectiveness of an individually addressable printhead 16 relative to its neighbors. If the first selected gas environment 36 of a certain absolute static pressure sensor 20 ceases to register changes in volume while the first selected gas environments of the absolute static pressure sensors 20 of all neighboring individually addressable printheads 16 continue to register fluctuations in volume, a defective or broken individually addressable printhead 16 can be detected during operation. Similarly, if the volume fluctuations of the first selected gas environment in one absolute static pressure sensor 20 indicate an inordinately large absolute static pressure change in one individually addressable printhead 16, a partial or total blockage of the microfluidic device 18 may have occurred. If the interrogation process is automated by the integration of an optical detection device, such as a CCD array (not pictured), then the printing apparatus 10 will have the ability to dynamically diagnose failure of certain individually addressable printheads 16 and direct the drive electronics 14 to correct for this failure. These benefits extend beyond the domain of inkjet printing into other applications. For example, microfluidic device releasing a steady dose of a drug through a microfluidic needle, a sudden drop in absolute static pressure (rise in volume) would be indicative of a blockage upstream, or a failure of the ink reservoir 12 or other pressure source driving the flow. Conversely, a rise in absolute static pressure would indicate a blockage downstream, causing the entire microfluidic device 18 to equilibrate to a higher absolute static pressure, since the majority of the absolute static pressure loss would occur across the blockage.

FIG. 3 depicts a second embodiment of the absolute static pressure sensor 20 shown in FIG. 1. Absolute static pressure is measured at multiple points along the microfluidic device 18. As with the first point of communication 32, a second point of communication 42 and any additional points of communication 52 should be kept as small as possible to minimize disruption to the flow in the microfluidic device 18. However, if the second point of communication 42 or any additional point of communication 52 is too small, the response time of the absolute static pressure sensor 20 will become large, since equilibration of the absolute static pressure sensor 20 will be limited by the rate at which fluid can flow through the second point of communication 42 or the additional point of communication 52, respectively. When the individually addressable printhead 16 is primed with a working fluid 30 (in this case ink) before use by the drive electronics 14, the microfluidic device 18 will fill with fluid. Depending on the pressure used to prime the absolute static pressure sensor 20, and the ambient pressure conditions upon filling, a certain amount of gas will be trapped within the first fluid conducting channel 34, the second fluid conducting channel 44, and any additional fluid conducting channels 54, forming a first selected gas environment 36 at the end of the first fluid conducting channel 34, a second selected gas environment 46 at the end of the second fluid conducting channel 44, and additional selected gas environments 56 at the ends of the additional fluid conducting channels 54.

For non-inkjet microfluidic devices, there will be an initial filling of the microfluidic device 18 generally accomplished by forcing the working fluid 30 through the microfluidic device 18 using an external pressure source. In order to use the absolute static pressure sensor 20 the initial volume of this first selected gas environment 36 must be determined by optically comparing the position of the first interface 38 to the first visual scale 40 while the absolute static pressure sensor 20 is under ambient pressure. Similarly, the initial volume of the second selected gas environment 46 must be determined by optically comparing the position of the second interface 48 to the second visual scale 50 while the absolute static pressure sensor 20 is under ambient pressure, and the initial volume of any additional selected gas environment 56 must be determined by optically comparing the position of the additional interface 58 to an additional visual scale 60 while the absolute static pressure sensor 20 is under ambient pressure.

Subsequently during microfluidic device operation, measurements of the first interface 38 position relative to the first visual scale 40 can be made to determine the volume of the first selected gas environment 36, measurements of the second interface 48 position relative to the second visual scale 50 can be made to determine the volume of the second selected gas environment 46, and measurements of any additional interface 58 position relative to the additional visual scale 60 can be made to determine the volume of the additional selected gas environment 56. Using the initial volume measured under ambient static conditions and an equilibrium operational volume measured while the microfluidic device 18 is in operation, the operational absolute static pressure can be determined. Since a fixed amount of gas is trapped within each of the selected gas environments 36, 46, and 56, the following relation holds for each operational absolute static pressure, P₂, under isothermal equilibrium conditions as a direct result of the ideal gas law: $P_{2} = {P_{1}\frac{V_{1}}{V_{2}}}$

where P₁ is the ambient absolute static pressure (measured with, for example, an external barometer) at which V₁, the initial volume, is measured and P₂ is the operational absolute static pressure at which V₂, the operational volume, is measured. As described above, more complex non-ideal and/or gas-specific relationships between P₁, V₁, P₂ and V₂ can be used for this calculation to model, for example diatomic vs. monatomic gases.

After calculating the absolute static pressure at each point of communication 32, 42 or 52, the static pressure differential from one point of communication 32, 42 or 52, to another can be determined by subtracting the calculated absolute static pressures from one another. By dividing the static pressure differential between two points of communication 32, 42 or 52, by the distance between the two points of communication 32, 42 or 52, an average static pressure gradient between the two absolute static pressure sensors 20 can be determined. Using the physical properties of the working fluid in combination with predetermined formulae for laminar flow in microfluidic channels with various geometrical cross sections (see, for example, Viscous Fluid Flow by Frank White, second edition, pp. 119-121) the flow rate can then be determined. For example, for a microfluidic channel with rectangular cross-section (a common cross-section for micromachined microfluidic channels), the flow rate would be determined by the following equation: $Q = {\frac{4{ba}^{3}}{3\mu}{\left( \frac{{- \Delta}\quad P}{\Delta \quad L} \right)\left\lbrack {1 - {\frac{192a}{\pi^{5}b}{\sum\limits_{{i = 1},3,5,\ldots}^{\infty}\quad \frac{\tan \quad {h\left( {{\pi}\quad {b/2}a} \right)}}{i^{5}}}}} \right\rbrack}}$

where Q is the flow rate of a working fluid of absolute viscosity μ through a rectangular microfluidic channel of width 2 a and height 2 b, and ΔP is the measured static pressure differential between two points of communication 32, 42, and/or 52 separated by a distance ΔL. Furthermore, by performing this calculation between several combinations of points of communication, the accuracy of the flow rate calculation can be confirmed. This capability also lends itself to analysis of microfluidic device clogging, localization of the region of failure of the microfluidic device 18. When the absolute static pressure downstream of a particular region drops while the upstream absolute static pressure remains constant, the source of the static pressure differential must be between the upstream and downstream absolute static pressure sensor.

FIG. 4 depicts a third embodiment of the absolute static pressure sensor 20 shown in FIG. 1, that measures absolute static pressure over several pressure ranges at its single first point of communication 30 with the microfluidic device 18. By varying the ratio of the volume of the selected gas environments 36, 46 or 56 to the cross-sectional areas of their respective fluid conducting channels 34, 44 or 54, an absolute static pressure sensor 20 is created that can measure absolute static pressure over a variety of ranges, since the displacement of each interface 38, 48 or 58 is determined by the following relationship: ${\Delta \quad x} = {\frac{{- \Delta}\quad V}{A} = {\frac{\Delta \quad P}{P_{1}}\frac{V_{2}}{A}}}$

where Δx is the change in interface 38, 48 or 58 position, ΔV is the change in the volume of the corresponding selected gas environment 36, 46 or 56, and A is the cross-sectional area of the microfluidic channel. When the absolute static pressure sensor 20 experiences small changes of the absolute static pressure from ambient static pressure in the working fluid 30 at the first point of communication 32, the volume of the selected gas environment 36, 46 or 56 will show a correspondingly small change in volume.

In the embodiment depicted in FIG. 4, the displacement from equilibrium of the first fluid interface 38 would be less than the displacement of the second fluid interface 48, which in turn would be less than the displacement of the additional interface 58. This can be magnified by using a larger additional selected gas environment 56, so that the change in volume ΔV is more evident, and/or by making the additional fluid conducting channel 54 narrower, so that the cross-sectional area A of the additional fluid conducting channel 54 decreases, increasing the linear displacement Δx of the additional interface. Thus, a narrow fluid conducting channel 34, 44 or 54 and a large selected gas environment 36, 46 or 56 are advantaged in accurately measuring a small change in absolute static pressure, which might be undetectable in a wider fluid conducting channel 34, 44 or 54 with a smaller selected gas environment 36, 46 or 56. However, the same sensor would not be able to measure large absolute static pressure changes, since the change in volume would compress the selected gas environment 36, 46 or 56 to a point where the interface 38, 48 or 58 position could no longer be measured on the visual scale 40, 50 or 60. By using additional sensors in communication with the microfluidic device 18 at the same point of communication 32, the higher absolute static pressure can be measured on a coarser scale. Thus, an absolute static pressure sensor 20 combining a number of fluid conducting channels 34, 44 or 54 and selected gas environments 36, 46 or 56 with different volume to cross-sectional area ratios can measure absolute static pressure over a variety of pressure ranges at the same point of communication, thereby increasing the effective absolute static pressure range of the sensor.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

PARTS LIST

10 Printing apparatus

12 Ink reservoir

14 Drive electronics

16 Individually addressable printhead

18 Microfluidic device

20 Absolute static pressure sensor

22 Computer

24 Data

26 Transport Mechanism

28 Receiver

30 Working Fluid

32 First point of communication

34 First fluid conducting channel

36 First selected gas environment

38 First interface

40 First visual scale

42 Second point of communication

44 Second fluid conducting channel

46 Second selected gas environment

48 Second interface

50 Second visual scale

52 Additional point of communication

54 Additional fluid conducting channel

56 Additional selected gas environment

58 Additional interface

60 Additional visual scale 

What is claimed is:
 1. A method of measuring absolute static pressure at one or more positions along the wall of a microfluidic device transporting a working fluid that is immiscible in a first selected gas environment, comprising the steps of: (a) providing a first fluid conducting channel having an atmosphere provided by the first selected gas environment under a sealed environment and in communication with the microfluidic device at a first point of communication; (b) providing a first visual scale adjacent to the first fluid conducting channel; and (c) transporting the working fluid under pressure conducted by the microfluidic device into the first fluid conducting channel such that the volume transported into such first fluid conducting channel varies depending upon the absolute static pressure of the working fluid at the first point of communication, whereby the absolute static pressure at the first point of communication is visually determined depending on the position of the interface of the working fluid and the first selected gas environment in the first fluid conducting channel when compared with the first visual scale.
 2. The method of claim 1 further including providing a second fluid conducting channel and a second visual scale adjacent to the second fluid conducting channel and in communication with the microfluidic device at a second point of communication whereby the static pressure differential in the microfluidic device between the first and second points of communication can be visually determined.
 3. The method of claim 1 further including at least one additional fluid conducting channel in direct communication with the first fluid conducting channel at an additional point of communication and providing an atmosphere by an additional selected gas in a sealed environment for such additional fluid conducting channel and an additional visual scale adjacent to the additional fluid conducting channel and wherein the additional selected gas environment associated with the additional fluid conducting channel has a different volume to cross-sectional area ratio than the first selected gas environment in the first fluid conducting channel.
 4. The method according to claim 1 wherein the microfluidic device is a channel, reservoir, chamber, conduit, pump, valve, mixer, needle or a system incorporating any combination of these.
 5. The method according to claim 1 wherein the first selected gas is air, nitrogen, argon, neon, helium, oxygen, hydrogen, water vapor, carbon monoxide, carbon dioxide, or a miscible combination of these.
 6. The method according to claim 1 wherein the working fluid is water, ink, ethylene glycol, methanol, ethanol, isopropanol, oil, acetone, an organic solvent, a biological fluid, a solution of a chemical analyte, or a miscible combination of these.
 7. A method of measuring flow rate between two or more positions along a microfluidic channel transporting a working fluid that is immiscible in a first selected gas environment, comprising the steps of: (a) providing a first fluid conducting channel having an atmosphere provided by the first selected gas environment under a sealed environment and in communication with the microfluidic channel at a first point of communication; (b) providing a first visual scale adjacent to the first fluid conducting channel; and (c) transporting the working fluid under pressure conducted by the microfluidic channel into the first fluid conducting channel such that the volume transported into such first fluid conducting channel varies depending upon the absolute static pressure of the working fluid in the microfluidic channel at the first point of communication, whereby the absolute static pressure at the first point of communication is visually determined depending on the position of the interface of the working fluid and the first selected gas environment in the first fluid conducting channel when compared with the first visual scale; (d) providing a second fluid conducting channel having an atmosphere provided by the second selected gas under a sealed environment and in communication with the microfluidic channel at a second point of communication; (e) providing a second visual scale adjacent to the second fluid conducting channel; (f) transporting the working fluid under pressure conducted by the microfluidic channel into the second fluid conducting channel such that the volume transported into such second fluid conducting channel varies depending upon the absolute static pressure of the working fluid in the microfluidic channel at the second point of communication, whereby the absolute static pressure at the second point of communication can be visually determined depending on the position of the interface of the working fluid and the second selected gas environment in the second fluid conducting channel when compared with the second visual scale; (g) providing an additional fluid conducting channel having an atmosphere provided by an additional selected gas under a sealed environment and in communication with the microfluidic channel at an additional point of communication; (h) providing an additional visual scale adjacent to the additional fluid conducting channel; (i) transporting the working fluid under pressure conducted by the microfluidic channel into the additional fluid conducting channel such that the volume transported into such additional fluid conducting channel varies depending upon the absolute static pressure of the working fluid in the microfluidic channel at the additional point of communication, whereby the absolute static pressure at the additional point of communication can be visually determined depending on the position of the interface of the working fluid and the additional selected gas environment in the additional fluid conducting channel when compared with the additional visual scale; (j) calculating the average static pressure gradient between any two of the points of communication by dividing the difference in absolute static pressure measured at the two points of communication by the separation distance between the two points of communication; and (k) calculating the flow rate of the working fluid through the microfluidic channel between any two of the points of communication from the average static pressure gradient between any two of the points of communication and the physical properties of the working fluid using predetermined formulae for laminar flow in microchannels with various cross-sections.
 8. The method of claim 7 further including at least one additional fluid conducting channel in direct communication with the first fluid conducting channel at an additional point of communication and providing an atmosphere by an additional selected gas environment in a sealed environment for such additional fluid conducting channel and an additional visual scale adjacent to the additional fluid conducting channel and wherein the additional selected gas environment associated with the additional fluid conducting channel has a different volume to cross-sectional area ratio than the first selected gas environment in the first fluid conducting channel.
 9. The method according to claim 7 wherein the first selected gas is air, nitrogen, argon, neon, helium, oxygen, hydrogen, water vapor, carbon monoxide, carbon dioxide, or a miscible combination of these.
 10. The method according to claim 7 wherein the working fluid is water, ink, ethylene glycol, methanol, ethanol, isopropanol, oil, acetone, an organic solvent, a biological fluid, a solution of a chemical analyte, or a miscible combination of these. 